Method and apparatus for characterizing materials by using a mechanical resonator

ABSTRACT

A method and apparatus for measuring properties of a liquid composition includes a mechanical resonator, such as a thickness shear mode resonator or a tuning fork resonator, connected to a measurement circuit. The measurement circuit provides a variable frequency input signal to the tuning fork, causing the mechanical resonator to oscillate. To test the properties of a liquid composition, the mechanical resonator is placed inside a sample well containing a small amount of the liquid. The input signal is then sent to the mechanical resonator and swept over a selected frequency range, preferably less than 1 MHz to prevent the liquid being tested from exhibiting gel-like characteristics and causing false readings. The mechanical resonator&#39;s response over the frequency range depends on various characteristics of the liquid being tested, such as the temperature, viscosity, and other physical properties. Particular mechanical resonators, such as tuning fork resonators, can also be used to measure a liquid composition&#39;s electrical properties, such as the dielectric constant and conductivity, because the tuning fork&#39;s structure allows a high degree of electrical coupling between the tuning fork and the surrounding liquid. The mechanical resonator can be covered with a coating to impart additional special detection properties to the resonator, and multiple resonators can be attached together as a single sensor to obtain multiple frequency responses. The invention is particularly suitable for combinatorial chemistry applications, which require rapid analysis of chemical properties for screening.

RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of commonlyassigned, copending U.S. application Ser. No. 08/946,921, filed Oct. 8,1997, the disclosure of which is incorporated by reference.

TECHNICAL FIELD

[0002] The present invention is directed to using mechanical oscillatorsfor measuring various properties of fluids (including both liquids andvapors), and more particularly to a method and system using a mechanicaloscillator (resonator) for measuring physical, electrical and/orchemical properties of a fluid based on the resonator's response in thefluid to a variable frequency input signal.

BACKGROUND ART

[0003] Companies are turning to combinatorial chemistry techniques fordeveloping new compounds having novel physical and chemical properties.Combinatorial chemistry involves creating a large number of chemicalcompounds by reacting a known set of starting chemicals in all possiblecombinations and then analyzing the properties of each compoundsystematically to locate compounds having specific desired properties.See, for example, U.S. patent application Ser. No. 08/327,513 (publishedas WO 96/11878), filed Oct. 18, 1994, entitled “The CombinatorialSynthesis of Novel Materials”, the disclosure of which is incorporatedby reference.

[0004] The virtually endless number of possible compounds that can becreated from the Periodic Table of Elements requires a systematicapproach to the synthesizing and screening processes. Thus, any systemthat can analyze each compound's properties quickly and accurately ishighly desirable. Further, such a system would be useful in anyapplication requiring quick, accurate measurement of a liquid'sproperties, such as in-line measurement of additive concentrations ingasoline flowing through a conduit or detection ofenvironmentally-offending molecules, such as hydrogen sulfide, flowingthrough a smokestack.

[0005] It is therefore an object of the invention to measuresimultaneously both the physical and the electrical properties of afluid composition using a mechanical resonator device.

[0006] It is also an object of the invention to detect differencesclearly between two or more compounds in a fluid composition by using amechanical resonator device to measure a composition's physical andelectrical properties.

[0007] It is a further object of the invention to use a mechanicalresonator device to monitor and measure a physical or chemicaltransformation of a fluid composition.

[0008] It is also an object of the invention to use a mechanicalresonator device to detect the presence of a specific material in afluid.

SUMMARY OF THE INVENTION

[0009] The present invention includes a method for measuring a propertyof a fluid composition using a tuning fork resonator, the methodcomprising:

[0010] placing the tuning fork resonator in the fluid composition suchthat at least a portion of the tuning fork resonator is submerged in thefluid composition;

[0011] applying a variable frequency input signal to a measurementcircuit coupled with the tuning fork resonator to oscillate the tuningfork resonator;

[0012] varying the frequency of the variable frequency input signal overa predetermined frequency range to obtain a frequency-dependentresonator response of the tuning fork resonator; and

[0013] determining the property of the fluid composition based on theresonator response.

[0014] The method can also measure a plurality of fluid compositions,wherein the fluid compositions are liquid compositions, using aplurality of tuning fork resonators, wherein the method furthercomprises:

[0015] providing an array of sample wells;

[0016] placing each of said plurality of liquid compositions in aseparate sample well;

[0017] placing at least one of said plurality of tuning fork resonatorsin at least one sample well;

[0018] applying a variable frequency input signal to a measurementcircuit coupled with each tuning fork resonator in said at least onesample wells to oscillate each tuning fork resonator associated witheach of said at least one sample well;

[0019] varying the frequency of the variable frequency input signal overa predetermined frequency range to obtain a frequency-dependentresonator response of each tuning fork resonator associated with said atleast one sample well; and

[0020] analyzing the resonator response of each tuning fork resonatorassociated with said at least one sample well to measure a property ofeach liquid composition in said at least one sample well.

[0021] Accordingly, the present invention is directed primarily to amethod using a mechanical piezoelectric quartz resonator (“mechanicalresonator”) for measuring physical and electrical properties, such asthe viscosity density product, the dielectric constant, and theconductivity of sample liquid compositions in a combinatorial chemistryprocess. The detailed description below focuses primarily on thicknessshear mode (“TSM”) resonators and tuning fork resonators, but othertypes of resonators can be used, such as tridents, cantilevers, torsionbars, bimorphs, or membrane resonators. Both the TSM resonator and thetuning fork resonator can be used to measure a plurality of compounds ina liquid composition, but the tuning fork resonator has desirableproperties that make it more versatile than the TSM resonator.

[0022] The mechanical resonator is connected to a measuring circuit thatsends a variable frequency input signal, such as a sinusoidal wave, thatsweeps over a predetermined frequency range, preferably in the 25-30 kHzrange for the tuning fork resonator and in a higher range for the TSMresonator. The resonator response over the frequency range is thenmonitored to determine selected physical and electrical properties ofthe liquid being tested. Although both the TSM resonator and the tuningfork resonator can be used to test physical and electrical properties,the tuning fork resonator is an improvement over the TSM resonatorbecause of the tuning fork's unique response characteristics and highsensitivity.

[0023] Both the TSM resonator and the tuning fork resonator can be usedin combinatorial chemistry applications according to the presentinvention. The small size and quick response of the tuning forkresonator in particular makes it especially suitable for use incombinatorial chemistry applications, where the properties of a vastnumber of chemicals must be analyzed and screened in a short timeperiod. In a preferred embodiment, a plurality of sample wellscontaining a plurality of liquid compositions are disposed on an array.A plurality of TSM or tuning fork resonators are dipped into the liquidcompositions, preferably one resonator per composition, and thenoscillated via the measuring circuit. Because the resonatingcharacteristics of both the TSM resonator and the tuning fork resonatorvirtually eliminate the generation of acoustic waves, the size of thesample wells can be kept small without the concern of acoustic wavesreflecting from the walls of the sample wells. In practice, the tuningforks can be oscillated at a lower frequency range than TSM resonators,making the tuning forks more applicable to real-world applications andmore suitable for testing a wide variety of compositions, including highmolecular weight liquids.

[0024] In another embodiment of the invention, the mechanical resonatoris coated with a material to change the resonator's characteristics. Thematerial can be a general coating to protect the resonator fromcorrosion or other problems affecting the resonator's performance, or itcan be a specialized “functionalization” coating that changes theresonator's response if a selected substance is present in thecomposition being tested by the resonator.

[0025] To obtain a more complete range of characteristics for a selectedfluid composition, multiple resonators having different resonatorcharacteristics can be connected together as a single sensor formeasuring the fluid composition. The resonator responses from all of theresonators in the sensor can then be correlated to obtain additionalinformation about the composition being tested. By using resonatorshaving different characteristics, the fluid composition can be testedover a wider frequency range than a single resonator. Alternatively, asingle resonator that can be operated in multiple mechanical modes (e.g.shear mode, torsion mode, etc.) can be used instead of the multipleresonators. The resonator responses corresponding to each mode would becorrelated to obtain the additional information about the composition.

[0026] The mechanical resonator system of the present invention,particularly a system using the tuning fork resonator, can also be usedto monitor changes in a particular liquid by keeping the resonator inthe liquid composition as it undergoes a physical and/or chemicalchange, such as a polymerization reaction. The invention is not limitedto measuring liquids, however; the quick response of the tuning forkresonator makes it suitable for measuring the composition of fluidcompositions, both liquid and vaporous, that are flowing through aconduit to monitor the composition of the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIGS. 1a and 1 b are cross-sectional views of a TSM resonatorplate and a tuning fork resonator tine used in preferred embodiments ofthe present invention, respectively;

[0028]FIG. 2 is a block diagram illustrating an example of a compositiontesting system of the present invention;

[0029]FIG. 3 is a representative diagram illustrating oscillationcharacteristics of the tuning fork resonator used in a preferredembodiment of the present invention;

[0030]FIGS. 4a and 4 b are simplified schematic diagrams illustrating atuning fork resonator connection with the measurement circuit in apreferred embodiment of the present invention;

[0031]FIG. 4c illustrates a sample response of the representativecircuit shown in FIG. 4b;

[0032]FIGS. 5a and 5 b are examples of traces comparing the frequencyresponses of the TSM resonator and the tuning fork resonator of thepresent invention, respectively;

[0033]FIGS. 6a and 6 b are examples of graphs illustrating therelationship between the viscosity density product and the equivalentserial resistance of the TSM resonator and the tuning fork resonator ofthe present invention, respectively;

[0034]FIGS. 7a and 7 b are examples of graphs illustrating therelationship between the dielectric constant and the equivalent parallelcapacitance of the TSM resonator and the tuning fork resonator of thepresent invention, respectively;

[0035]FIGS. 8a and 8 b are examples of graphs illustrating therelationship between the molecular weight of a sample composition andthe equivalent serial resistance of the TSM resonator and the tuningfork resonator of the present invention, respectively, in apolymerization reaction;

[0036]FIGS. 9a and 9 b illustrate another embodiment of the inventionusing a resonator that is treated with a coating for targeting detectionof specific chemicals; and

[0037]FIGS. 10a, 10 b and 10 c illustrate examples of different multipleresonator sensors of yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The method and apparatus of the present invention focuses onusing a mechanical resonator to generate and receive oscillations in afluid composition for testing its characteristics in a combinatorialchemistry process or other process requiring analysis of the fluidcomposition's physical and/or chemical properties. Although the detaileddescription focuses on combinatorial chemistry and the measurement of aliquid composition's characteristics, the invention can be used in anyapplication requiring measurement of characteristics of a fluidcomposition, whether the fluid is in liquid or vapor form. The fluidcomposition itself can be any type of fluid, such as a solution, aliquid containing suspended particulates, or, in some embodiments, evena vapor containing a particular chemical or a mixture of chemicals. Itcan also include a liquid composition undergoing a physical and/orchemical change (e.g. an increase in viscosity).

[0039] Mechanical resonators, such as thickness shear mode (TSM) quartzresonators 10, are used in the present invention for measuring variousphysical properties of fluid compositions, such as a liquid's viscosity,molecular weight, specific weight, etc., in a combinatorial chemistrysetting or other liquid measurement application. Referring to FIG. 1a,TSM resonators 10 usually have a flat, plate-like structure where aquartz crystal 12 is sandwiched in between two electrodes 14. Incombinatorial chemistry applications, the user first generates a“library”, or large collection, of compounds in a liquid composition.Normally, each liquid composition is placed into its own sample well. ATSM resonator 10 connected to an input signal source (not shown) isplaced into each liquid composition, and a variable frequency inputsignal is sent to each TSM resonator 10, causing the TSM resonator 10 tooscillate. The input signal frequency is swept over a predeterminedrange to generate a unique TSM resonator 10 response for each particularliquid. Because every compound has a different chemical structure andconsequently different properties, the TSM resonator 10 response will bealso be different for each compound. The TSM resonator response is thenprocessed to generated a visual trace of the liquid composition beingtested. An example of traces generated by the TSM resonator 10 formultiple liquid compositions is shown in FIG. 5a. Screening and analysisof each compound's properties can then be conducted by comparing thevisual traces of each compound with a reference and/or with othercompounds. In this type of application, the TSM resonator 10 serves bothas the wave source and the receiver.

[0040] Two types of waves can be excited in liquids: compression waves(also called acoustic waves), which tend to radiate a large distance, onthe order of hundreds of wavelengths, from the wave-generating source;and viscose shear waves, which decay almost completely only onewavelength away from the wave-generating source. In any liquid propertytesting, acoustic waves should be kept to a minimum because they willcreate false readings when received by the resonator due to their longdecay characteristics. For typical prior art ultrasonictransducers/resonators, the resonator oscillation creates acoustic wavesthat radiate in all directions from the resonator, bounce off the sidesof the sample well, and adversely affect the resonator response. As aresult, the resonator response will not only reflect the properties ofthe liquid being measured, but also the effects of the acoustic wavesreflecting from the walls of the sample well holding the liquid, therebycreating false readings. Using a sample well that is much greater thanthe acoustic wavelength does minimize the negative effects of acousticwaves somewhat, but supplying thousands of sample wells having suchlarge dimensions tends to be impractical.

[0041] TSM resonators 10 primarily generate viscose shear waves and aretherefore a good choice for liquid property measurement in combinatorialchemistry applications because they do not generate acoustic waves thatcould reflect off the sides of the sample wells and generate falsereadings. As a result, the sample wells used with the TSM resonators 10can be kept relatively small, making it feasible to construct an arrayof sample wells for rapid, simultaneous testing of many liquids. Thehigh stiffness of TSM resonators 10, however, require them to beoperated at relatively high frequencies, on the order of 8-10 MHz. Thisstiffness does not adversely affect measurement accuracy for manyapplications, though, making the TSM resonator an appropriate choice formeasuring numerous liquid compositions.

[0042] However, TSM resonators 10 can be somewhat insensitive to thephysical properties of certain liquids because the load provided by thesurrounding liquid is less than the elasticity of the resonator. Moreparticularly, the high operating frequencies of TSM resonators 10 makethem a less desirable choice for measuring properties of certain liquidcompositions, particularly high-molecular weight materials such aspolymers. When high frequency waves are propagated through highmolecular-weight liquids, the liquids tend to behave like gels becausethe rates at which such large molecules move correspond to frequenciesthat are less than that of the TSM resonator's oscillations. This causesthe TSM resonator 10 to generate readings that sometimes do not reflectthe properties at which the liquids will actually be used (mostmaterials are used in applications where the low-frequency dynamicresponse is most relevant). Although it would be more desirable tooperate the TSM resonator 10 at lower frequencies so that laboratoryconditions reflect real world conditions, the stiffness of the TSMresonator 10 and its resulting high operating frequencies can makeoperation at lower frequencies rather difficult. Further, even when theTSM resonator 10 can accurately measure a liquid's properties, thedifferences in the visual traces associated with different compositionsare relatively slight, making it difficult to differentiate betweencompositions having similar structures, as shown in FIG. 5a.

[0043] TSM resonators and other plate-type resonators, while adequate,may not always be the best choice for measuring the electricalcharacteristics, such as the dielectric constant, of the liquidcomposition being measured. As shown in FIG. 1a, the cross-section of aTSM resonator 10 has the same structure as a flat capacitor, resultingin relatively little coupling between the electric field of theresonator and the surrounding composition. While there can be enoughelectrical coupling between the resonator and the composition to measurethe composition's electrical properties, a greater amount of electricalcoupling is more desirable for increased measurement accuracy.Electrical coupling will be explained in greater detail below whencomparing the electrical characteristics between the TSM resonator 10and the tuning fork resonator 20.

[0044]FIGS. 1a and 1 b show a cross-section of a TSM resonator plate 10and a tuning fork tine 22, respectively. The tuning fork resonator 20 ispreferably made from a quartz crystal 24 and has two tines 22, asrepresented in FIG. 2, each tine having the quartz crystal center 24 andat least one electrode 26 connected to the quartz crystal 24. The tuningfork tines 22 in the preferred structure have a square or rectangularcross-section such that the quartz crystal center 24 of each tine hasfour faces. The electrodes 26 are then attached to each face of thequartz crystal center 24, as shown in FIG. 1b. The method and system ofthe present invention can use any type of tuning fork resonator, such asa trident (three-prong) tuning fork or tuning forks of different sizes,without departing from the spirit and scope of the invention.

[0045] The cross-sectional views of the TSM resonator 10 and the tuningfork resonator 20 shown in FIGS. 1a and 1 b also illustrate the relativedifferences between the electric coupling of each resonator with thesurrounding liquid. Referring to FIG. 1a, the structure of the TSMresonator 10 is very flat, making it close to a perfect capacitor whenit is placed in the liquid to be measured. As noted above, the quartzcrystal 12 in the TSM resonator 10 is sandwiched between two electrodes14, causing most of an electric field 16 to travel between the twoelectrodes through the quartz crystal 12. Because most of the electricfield 16 is concentrated within the quartz crystal 12 rather thanoutside of it, there is very little electric coupling between the TSMresonator 10 and the surrounding liquid except at the edges of theresonator 10. While there may be sufficient electrical coupling tomeasure the electrical properties, such as the conductivity ordielectric constant, of the liquid composition being tested, a greaterdegree of coupling is desirable to ensure more accurate measurement.

[0046] By comparison, as shown in Figure lb, the structure of eachtuning fork tine 22 allows much greater electrical coupling between thetine 22 and the surrounding liquid because the tuning fork tine'scross-sectional structure has a much different structure than a flatcapacitor. Because the tuning fork tine 22 is submerged within theliquid being tested, an electric field 27 associated with each tine 22does not concentrate in between the electrodes 24 or within the quartzcrystal 24, but instead interacts outside the tine 22 with thesurrounding liquid. This increased electrical coupling allows the tuningfork 20 to measure accurately the electrical properties of the liquid aswell as its physical properties, and it can measure both types ofproperties simultaneously if so desired.

[0047] One unexpected result of the tuning fork resonator 20 is itsability to suppress the generation of acoustic waves in a liquid beingtested, ensuring that the resonator's 20 physical response will be basedonly on the liquid's physical properties and not on acoustic waveinterference or the shape of the sample well holding the liquid. Asexplained above, TSM resonators 10 minimize excitation of acoustic wavesbecause it generates shear oscillations, which do not excite wavesnormal to the resonator's surface. As also explained above, however, theTSM resonator 10 requires high frequency operation and is not suitablefor many measurement applications, particularly those involvinghigh-molecular weight liquids.

[0048] Without wishing to be bound by any particular theory, theinventors believe that the tuning fork resonator 20 used in the presentinvention virtually eliminates the effects of acoustic waves withouthaving to increase the size of the sample wells to avoid wavereflection. Tuning fork resonators 20, because of their shape and theirorientation in the liquid being tested, contain velocity componentsnormal to the vibrating surface. Thus, it was assumed in the art thattuning fork resonators were unsuitable for measuring liquid propertiesbecause they would generate acoustic waves causing false readings. Inreality, however, tuning fork resonators 20 are very effective insuppressing the generation of acoustic waves for several reasons. First,the preferred size of the tuning fork resonator 20 used in the inventionis much smaller than the wavelength of the acoustic waves that arenormally generated in a liquid, as much as one-tenth to one-hundredththe size. Second, as shown in FIG. 3, the tines 22 of the tuning forkresonator 20 oscillate in opposite directions, each tine 22 acting as aseparate potential acoustic wave generator. In other words, the tines 22either move toward each other or away from each other. Because the tines22 oscillate in opposite directions and opposite phases, however, thewaves that end up being generated locally by each tine 22 tend to canceleach other out, resulting in virtually no acoustic wave generation fromthe tuning fork resonator 22 as a whole.

[0049] A simplified diagram of one example of the inventive mechanicalresonator 20 system is shown in FIG. 2. Although the explanation of thesystem focuses on using the tuning fork resonator 20, the TSM resonator10 described above can also be used for the same purpose. To measure theproperty of a given liquid, the tuning fork resonator 20 is simplysubmerged in the liquid to be tested. A variable frequency input signalis then sent to the tuning fork resonator using any known means tooscillate the tuning fork, and the input signal frequency is swept overa predetermined range. The tuning fork resonator's response is monitoredand recorded. In the example shown in FIG. 2, the tuning fork resonator20 is placed inside a well 26 containing a liquid to be tested. Thisliquid can be one of many liquids for comparison and screening or it cansimply be one liquid whose properties are to be analyzed independently.Further, if there are multiple liquids to be tested, they can be placedin an array and measured simultaneously with a plurality of tuning forkresonators to test many liquids in a given amount of time. The liquidcan also be a liquid that is undergoing a polymerization reaction or aliquid flowing through a conduit.

[0050] The tuning fork resonator 20 is preferably coupled with a networkanalyzer 28, such as a Hewlett-Packard 8751A network analyzer, whichsends a variable frequency input signal to the tuning fork resonator 20to generate the resonator oscillations and to receive the resonatorresponse at different frequencies. The resonator output then passesthrough a high impedance buffer 30 before being measured by a wide bandreceiver 32. The invention is not limited to this specific type ofnetwork analyzer, however; any other analyzer that generates andmonitors the resonator's response over a selected frequency range can beused without departing from the scope of the invention. For example, asweep generator and AC voltmeter can be used in place of the networkanalyzer.

[0051] An equivalent circuit of the tuning fork resonator 20 and itsassociated measurement circuit is represented in FIGS. 4a and 4 b . FIG.4a represents an illustrative tuning fork resonator system that measuresa liquid's viscosity and dielectric constant simultaneously, while FIG.4b represents a tuning fork resonator system that can also measure aliquid's conductivity as well. Referring to FIG. 4a, the measurementcircuit includes a variable frequency input signal source 42, and theresonator equivalent circuit 43 contains series capacitor Cs, resistorRs, inductor L, and parallel capacitor Cp. The resonator equivalentcircuit 43 explicitly illustrates the fact that the quartz crystal 24 inthe tuning fork resonator 20 acts like a capacitor Cp. Therepresentative circuit 40 also includes input capacitor Cin, inputresistor Rin and an output buffer 44.

[0052] The representative circuit shown in FIG. 4b adds a parallelresistor Rp in parallel to capacitor Cp to illustrate a circuit thatmeasures conductivity as well as dielectric constant and viscosity,preferably by comparing the equivalent resistance found in a givenliquid with a known resistance found via calibration. These conceptswill be explained in further detail below with respect to FIGS. 5a-b, 6a-b, 7 a-b, and 8 a-b. Rp represents the conductivity of the liquidbeing tested. The resistance can be calibrated using a set of liquidshaving known conductivity and then used to measure the conductivity of agiven liquid. For example, FIG. 4c shows a sample trace comparing theresonator response in pure toluene and in KaBr toluene solution. Aliquid having greater conductivity tends to shift the resonator responseupward on the graph, similar to liquids having higher dielectricconstants. However, unlike liquids with higher dielectric constants, aliquid having greater conductivity will also cause the resonatorresponse to level out somewhat in the frequency sweep, as can be seen inthe upper trace 45 between 30 and 31.5 kHz. In the example shown in FIG.4c, the difference between the upper trace 45 and the lower trace 46indicates that the equivalent resistance Rp caused by the additionalKaBr in solution was about 8 mega-ohms.

EXPERIMENTAL EXAMPLES

[0053]FIGS. 5a-b, 6 a-b, 7 a-b and 8 a-b are examples demonstrating theeffectiveness of the invention. These figures show some differencesbetween the frequency responses, for various liquid compositions, of theplate-type TSM resonator 10 and the tuning fork resonator 20. FIGS. 5a,6 a, 7 a and 8 a are examples using the TSM resonator 10, and FIGS. 5b,6 b, 7 b and 8 b are examples using the tuning fork resonator 20.

[0054] The experimental conditions for generating the example tuningfork resonator traces in FIGS. 5b, 6 b, 7 b, and 8 b are describedbelow. The experimental conditions for generating the comparative TSMresonator traces in FIGS. 5a, 6 a, 7 a and 8 a are generally similar to,if not the same as, the conditions for the tuning fork resonator exceptfor, if needed, minor modifications to accommodate the TSM resonator'sparticular geometry. Therefore, for simplicity and clarity, the TSMresonator's particular experimental conditions will not be describedseparately.

[0055] All of the solvents, polymers and other chemicals used in theillustrated examples were purchased from Aldrich, and the polymersolutions were made according to standard laboratory techniques. Drypolymers and their corresponding solvents were weighed using standardbalances, and the polymer and solvent were mixed until the polymerdissolved completely, creating a solution having a known concentration.The solutions were delivered to and removed from a 30 ul stainless steelcylindrical measurement well that is long enough to allow a tuning forkresonator to be covered by liquid. Liquid delivery and removal to andfrom the well was conducted via a pipette or syringe.

[0056] Before any experiments were conducted with the solutions, thetuning fork resonator response in air was measured as a reference. Theactual testing processes were conducted in a temperature-controlledlaboratory set at around 20 degrees Centigrade. Once the liquid wasdelivered to the well, the tuning fork was placed in the well and thesystem was left alone to allow the temperature to stabilize.Alternatively, the tuning fork can be built into a wall portion or abottom portion of the well with equally accurate results. The tuningfork was then oscillated using the network analyzer. The resonatorresponse was recorded during each measurement and stored in a computermemory. The measured response curve was fitted to a model curve using anequivalent circuit, which provided specific values for the equivalentcircuit components described above with respect to FIGS. 4a and 4 b andthe traces in FIGS. 6a through 8 b.

[0057] After the measurement of a given solution was completed, theresonator was kept in the well and pure solvent was poured inside thewell to dissolve any polymer residue or coating in the well and on thetuning fork. The well and tuning fork were blown dry using dry air, andthe tuning fork response in air was measured again and compared with theinitial tuning fork measurement to ensure that the tuning fork wascompletely clean; a clean tuning fork would give the same response asthe initial tuning fork response. Note that the above-describedexperimental conditions are described only for purposes of illustrationand not limitation, and those of ordinary skill in the art wouldunderstand that other experimental conditions can be used withoutdeparting from the scope of the invention.

[0058] Although both the TSM resonator 10 and the tuning fork resonator20 are considered to be part of the method and system of the presentinvention, the tuning fork resonator 20 has wider application than theTSM resonator 10 and is considered by the inventors to be the preferredembodiment for most measurement applications because of its sensitivity,availability and relatively low cost. For example, note that in FIGS. 5aand 5 b, the frequency sweep for the TSM resonator 10 is in the 8 MHzrange, while the frequency sweep for the tuning fork resonator 20 of thepresent invention is in the 25-30 kHz range, several orders of magnitudeless than the TSM resonator frequency sweep range. This increases theversatility and applicability of the tuning fork resonator 20 formeasuring high molecular weight liquids because the operating frequencyof the tuning fork resonator 20 is not high enough to make highmolecular weight liquids act like gels. Further, because mostapplications for the solutions are lower frequency applications, thelaboratory conditions in which the liquid compositions are tested usingthe tuning fork resonator 20 more closely correspond with real-worldconditions.

[0059] Also, the operating frequency of the tuning fork resonator 20varies according to the resonator's geometry; more particularly, theresonance frequency of the tuning fork 20 depends on the ratio betweenthe tine cross-sectional area and the tine's length. Theoretically, itis possible to construct a tuning fork resonator 20 of any length for agiven frequency by changing the tuning fork's cross-sectional area tokeep the ratio between the length and the cross-section constant. Inpractice, however, tuning fork resonators 20 are manufactured fromquartz wafers having a few selected standard thicknesses. Therefore, thecross-sectional area of the tuning fork 20 tends to be limited based onthe standard quartz wafer thicknesses, forcing the manufacturer tochange the tuning fork's resonating frequency by changing the tinelength. These manufacturing limitations must be taken into account whenselecting a tuning fork resonator 20 that is small enough to fit inminimal-volume sample wells (because the chemicals used are quiteexpensive) and yet operates at a frequency low enough to prevent thetested liquids from acting like gels. Of course, in other applications,such as measurement of liquids in a conduit or in other containers, theoverall size of the tuning fork resonator 20 is not as crucial, allowinggreater flexibility in selecting the size and dimensions of the tuningfork resonator 20. Selecting the actual tuning fork dimensions anddesigning a tuning fork resonator in view of manufacturing limitationsare tasks that can be conducted by those of skill in the art afterreviewing this specification.

[0060] Referring to FIGS. 5a and 5 b, the solutions used as examples inFIGS. 5a and 5 b have somewhat similar structures and weights. As aresult, the TSM resonator responses for each solution, shown in FIG. 5a,create very similar traces in the same general range. Because the tracesassociated with the TSM resonator 10 overlap each other to such a greatextent, it is difficult to isolate and compare the differences betweenthe responses associated with each solution. By comparison, as shown inFIG. 5b, the increased sensitivity of the tuning fork resonator 20causes small differences in the chemical structure to translate intosignificant differences in the resonator response. Because the tracesgenerated by the tuning fork resonator 20 are so distinct and spacedapart, they are much easier to analyze and compare.

[0061] Using a tuning fork resonator 20 to measure properties of liquidsalso results in greater linearity in the relationship between the squareroot of the product of the liquid's viscosity density and the equivalentserial resistance Rs (FIGS. 6a and 6 b) as well as in the relationshipbetween the dielectric constant and the equivalent parallel capacitanceCp (FIGS. 7a and 7 b) compared to TSM resonators 10. For example, therelationship between the liquid viscosity and serial resistance for atuning fork resonator 20, as shown in FIG. 6b, is much more linear thanthat for the TSM resonator, as shown in FIG. 6a.

[0062] Similarly, the relationship between the dielectric constant andthe equivalent parallel capacitance is more linear for a tuning forkresonator 20, as shown in FIGS. 7a and 7 b. This improved linearrelationship is primarily due to the relatively low frequencies at whichthe tuning fork resonator 20 operates; because many liquids exhibitdifferent behavior at the operating frequencies required by the TSMresonator 10, the TSM resonator 10 will tend not to generate testingresults that agree with known data about the liquids' characteristics.

[0063]FIGS. 8a and 8 b illustrate sample results from real-timemonitoring of polymerization reactions by a TSM resonator and a tuningfork resonator, respectively. The graphs plot the equivalent resistanceRs of the resonators oscillating in 10 and 20 mg/ml polystyrene-toluenesolutions versus the average molecular weight of polystyrene. Asexplained above, high molecular weight solutions often exhibit differentphysical characteristics, such as viscosity, at higher frequencies.

[0064] The size and shape of the TSM resonator 10 make the resonatorsuitable, but not as accurate, for real-time monitoring ofpolymerization reactions compared with the tuning fork resonator 20.This is because the TSM resonator's high operating frequency reduces theaccuracy of measurements taken when the molecular weight of thepolymerizing solution increases. As shown in FIG. 8a, a high operatingfrequency TSM resonator is not very sensitive in monitoring themolecular weight of the polystyrene solution used in the illustratedexample. A tuning fork resonator, by contrast, has greater sensitivityto the molecular weight of the solution being measured, as shown in FIG.8b. This sensitivity and accuracy makes it possible, for many reactions,to estimate the amount of converted solution in the polymerizationreaction and use the conversion data to estimate the average molecularweight of the polymer being produced.

[0065] Although the above-described examples describe using a TSM or atuning fork resonator without any modifications, the resonator can alsobe treated with a “functionality” (a specialized coating) so that it ismore sensitive to certain chemicals. The resonator may also be treatedwith a general coating to protect the resonator from corrosion or otherproblems that could impede its performance. A representative diagram ofan embodiment having a functionalized resonator is shown in FIGS. 9a and9 b. Although FIGS. 9a and 9 b as well as the following descriptionfocuses on coating or functionalizing a tuning fork resonator, any othermechanical resonator can also be used without departing from the scopeof the invention.

[0066] The tuning fork resonator 20 can be coated with a selectedmaterial to change how the resonator 20 is affected by a fluidcomposition (which, as explained earlier, includes both liquid and vaporcompositions). As mentioned above, one option is a general coating forproviding the tuning fork resonator 20 with additional properties suchas corrosion resistance, chemical resistance, electrical resistance, andthe like. Another option, as noted above, is using a “functionality”,which coats the tines with materials that are designed for a specificapplication, such as proteins to allow the tuning fork resonator 20 tobe used as a pH meter or receptors that attract specific substances inthe fluid composition to detect the presence of those substances. Thecoating or functionality can be applied onto the tuning fork resonator20 using any known method, such as spraying or dipping. Further, thespecific material selected for the coating or functionality will dependon the specific application in which the tuning fork resonator 20 is tobe used. J. Hlavay and G. G. Guilbault described various coating andfunctionalization methods and materials to adapt piezoelectric crystaldetectors for specific applications in “Applications of thePiezoelectric Crystal Detector in Analytical Chemistry,” AnalyticalChemistry, Vol. 49, No. 13, November 1977, p. 1890, incorporated hereinby reference. For example, applying different inorganic functionalitiesto the tuning fork resonator 20 allows the resonator to detectorganophosphorous compounds and pesticides.

[0067] An example of a tuning fork resonator that has undergone afunctionalization treatment is illustrated in FIGS. 9a and 9 b. FIG. 9arepresents a tuning fork tine 22 that has been treated by absorbing,coating, or otherwise surrounding the tine 22 with a functionalitydesigned to change the tuning fork's resonance frequency after beingexposed to a selected target chemical. In the illustrated example, thetuning fork tine 22 is covered with receptor molecules 90, representedin FIGS. 9a and 9 b by Y-shaped members, designed to bond with specifictarget molecules. Because the resonance frequency and the damping of thetuning fork resonator depends on the effective mass of the tine 22 andthe amount of “drag” of the tine 22 within the fluid, any change in thetine's mass or the amount of drag will change the tuning fork'sresonance response. More specifically, the resonance frequency of thetuning fork resonator is proportional to the square root of the inverseof the tuning fork's mass. An increase in the tuning fork's mass willtherefore reduce the tuning fork's resonance frequency.

[0068] This mass-frequency relationship is used to detect the presenceof a specific target chemical in a fluid composition in this example.When the functionalized tuning fork tine 22 is placed in a fluidcomposition containing the target chemical, the receptors 90 on thetuning fork tine 22 will chemically bond with molecules of the targetchemical 92, as shown in FIG. 9b. The resonance frequency of the tuningfork resonator will consequently decrease because of the increased massand the additional drag created by the additional molecules 92 attachedto the tuning fork tines 22 via the receptor molecules 90. Thus, whenscreening a plurality of fluid compositions to detect the presence of atarget chemical in any of them, only the fluid compositions containingthe target chemical will cause the tuning fork's resonance frequency tochange. Fluid compositions without the target chemical will not containmolecules that will bond with the receptor molecules 90 on the tuningfork tine 22, resulting in no resonance frequency change for thosefluids. Alternatively, the tuning fork tines 22 can be functionalizedwith a material that physically changes when exposed to molecules of aselected chemical such that the material changes the mechanical drag onthe tuning fork tine 22 when it is exposed to the selected chemical. Forexample, adding a hydrophobic or hydrophilic functionality to the tuningfork tine 22 allows the tine 22 to attract or repel selected substancesin the medium being analyzed, changing the mass or effective mass of thetuning fork and thereby changing its resonance frequency.

[0069] In yet another embodiment of the present invention, multiplemechanical resonators can be attached together in a single sensor tomeasure a wider range of responses for a given fluid composition, asshown in FIGS. 10a, 10 b and 10 c. The multiple resonator sensor can befabricated from a single quartz piece such that all of the resonatorsare attached together by a common base, as shown in the figures. Themulti-resonator sensor could also be attached to multiple frequencygenerating circuits, such as multiple network analyzers 28, to measureproperties of the fluid compositions over multiple frequency sweeps sothat the generated data can be correlated to obtain additionalinformation about the liquid compositions. Because different resonatorstructures are best suited for measurement over different frequencyranges and for materials having different characteristics, a sensorcombining a plurality of different resonators can provide a morecomplete representation of the fluid composition's characteristics overa wider frequency range than a single resonator. FIGS. 10a, 10 b and 10c show specific examples of possible multi-resonator configurations, butthose of skill in the art would understand that sensors having anycombination of resonators can be constructed without departing from thescope of the invention.

[0070]FIG. 10a illustrates one possible sensor 100 configurationcontaining both a tuning fork resonator 102 and a TSM resonator 104.This type of sensor 100 can be used to, for example, measure themechanical and electrical properties of very thick liquids such aspolymer resins and epoxies. This sensor 100 can also be used to monitora material as it polymerizes and hardens. For example, the sensor 100can be placed in a liquid composition containing urethane rubber in itsdiluted state so that the tuning fork 102 is used initially to measureboth the composition's density viscosity product and its dielectricconstant. As the rubber changes to a gel and finally to a solid, thesensor 100 can switch to using the TSM resonator 104 to measure therubber's mechanical properties, leaving the tuning fork resonator 102 tooperate as a dielectric sensor only.

[0071] A sensor 106 for observing a fluid composition over a widefrequency range is shown in FIG. 10b. High polydispersity polymersolutions are ideally measured over a wide frequency spectrum, but mostresonators have optimum performance within a relatively limitedfrequency range. By combining different resonators having differentresonance frequencies and different response characteristics, it ispossible to obtain a more complete spectrum of resonator responses foranalyzing the fluid's characteristics under many different conditions.For example, due to the wide spectrum of polydisperse solutionrelaxation times, it is generally predicted that high molecular weightcompositions will react at lower frequencies than lighter molecularweight compositions. By changing the temperature, observing thefrequency response of different resonators, and correlating thedifferent resonator responses, it is possible to obtain a more accuratepicture of a composition's relaxation spectrum than from a singleresonator.

[0072] A low frequency tuning fork resonator 108 and a high frequencytuning fork resonator 110 in one sensor will probably suffice for mostwide-frequency range measurements. For certain cases, however, theresonators in the multi-resonator sensor 106 can also include a tridenttuning fork resonator 112, a length extension resonator 114, a torsionresonator 116, and a TSM resonator 1 18, membrane oscillators, bimorphs,unimorphs, and various surface acoustic wave devices, as well as anycombination thereof, or even a single resonator structure than canoperate in multiple mechanical modes (e.g. compression mode, axial mode,torsion mode). Of course, not all of these resonators are needed forevery application, but those of skill in the art can select differentcombinations that are applicable to the specific application in whichthe sensor 106 will be used.

[0073] Alternatively, multiple resonators having the same structure butdifferent coatings and/or functionalities can be incorporated into onesensor 120, as shown in FIG. 10c. In this example, a plurality of tuningfork resonators 122, 124, 126 have the same structure but have differentfunctionalities, each functionality designed to, for example, bond witha different target molecule. The high sensitivity of the tuning forkresonators 122, 124, 126 makes them particularly suitable for“artificial noses” that can detect the presence of anenvironmentally-offending molecule, such as hydrogen sulfide or nitrousoxide, in industrial emissions. When the sensor 120 is used in such anapplication, one tuning fork resonator 122 can, for example, befunctionalized with a material designed to bond with hydrogen sulfidewhile another resonator 124 can be functionalized with a materialdesigned to bond with nitrous oxide. The presence of either one of thesemolecules in the fluid composition being tested will cause thecorresponding tuning fork resonator 122, 124 to change its resonancefrequency, as explained with respect to FIGS. 9a and 9 b.

[0074] The tuning fork resonators 122, 124, 126 can also befunctionalized with a polymer layer or other selective absorbing layerto detect the presence of specific molecules in a vapor. Because thetuning fork resonators 122, 124, 126 are highly sensitive to thedielectric constant of the surrounding fluid, the tuning fork resonators122, 124, 126 can easily detect changes in the dielectric constant ofthe fluid and recognize a set of solvents with different dielectricconstants in the fluid. This information, combined with other observableparameters, makes tuning fork resonators particularly adaptable for usein artificial noses.

[0075] The method and system of the present invention has been describedabove in the combinatorial chemistry context, but it is not limited tosuch an application. Because the resonators in the method and system ofthe present invention have high sensitivities and quick response times,it can be also be used for in-line monitoring of fluid compositionsflowing through conduits or pipelines. For example, the invention can beused in a feedback system to monitor properties of liquids flowingthrough a gas or oil pipeline to monitor and control the concentrationof additives in the gas or oil, or to detect the presence of impuritiesin water flowing through a water pipe. The additives or impurities willchange the physical and electrical characteristics of the liquid flowingthrough the conduit. A functionalized tuning fork resonator 20 canfurther detect the presence of a specific chemical in a fluidcomposition, whether it is a liquid or a vapor, and can be used tomonitor the presence of, for example, a known chemical pollutant in asmokestack. The high sensitivity and quick response time of theresonator, and the tuning fork resonator 20 in particular, makes ituniquely suitable for such an application. The circuitry and system usedto generate the visual traces from the resonator's response can be thesame as described above or be any other equivalent resonator analysissystem.

[0076] Further, although the above description focuses primarily onusing TSM resonators and tuning fork resonators, any other mechanicalresonators exhibiting similar characteristics can be used. Tridents,cantilevers, torsion bars, bimorphs, and/or membrane resonators can besubstituted for the TSM resonator or tuning fork resonator withoutdeparting from the scope of the claimed invention.

[0077] It should be understood that various alternatives to theembodiments of the invention described herein may be employed inpracticing the invention. It is intended that the following claimsdefine the scope of the invention and that the methods and apparatuswithin the scope of these claims and their equivalents be coveredthereby.

What is claimed is:
 1. A method for measuring a property of a fluidcomposition using a tuning fork resonator, the method comprising:placing the tuning fork resonator in the fluid composition such that atleast a portion of the tuning fork resonator is surrounded by the fluidcomposition; applying a variable frequency input signal to a measurementcircuit coupled with the tuning fork resonator to oscillate the tuningfork resonator; varying the frequency of the variable frequency inputsignal over a predetermined frequency range to obtain afrequency-dependent resonator response of the tuning fork resonator; anddetermining the property of the fluid composition based on the resonatorresponse.
 2. The method of claim 1 , wherein the fluid composition is aliquid composition, wherein a plurality of liquid compositions aremeasured by a plurality of tuning fork resonators, and wherein saidmethod further comprises: providing an array of sample wells; placingeach of said plurality of liquid compositions in a separate sample well;placing at least one of said plurality of tuning fork resonators in atleast one sample well; applying a variable frequency input signal to ameasurement circuit coupled with each tuning fork resonator in said atleast one sample wells to oscillate each tuning fork resonatorassociated with each of said at least one sample well; varying thefrequency of the variable frequency input signal over a predeterminedfrequency range to obtain a frequency-dependent resonator response ofeach tuning fork resonator associated with said at least one samplewell; and analyzing the resonator response of each tuning fork resonatorassociated with said at least one sample well to measure a property ofeach liquid composition in said at least one sample well.
 3. The methodof claim 2 , wherein said method measures a physical property of eachliquid composition in the array of sample wells.
 4. The method of claim2 , wherein the physical property measured by said method is selectedfrom the group consisting of specific weight, temperature and viscosity.5. The method of claim 2 , wherein said method measures an electricalproperty of each liquid composition in the array of sample wells.
 6. Themethod of claim 5 , wherein the electrical property measured by saidmethod is selected from the group consisting of dielectric constant andconductivity.
 7. The method of claim 2 , wherein said method measures anelectrical property and a physical property of each liquid compositionin the array of sample wells simultaneously.
 8. The method of claim 7 ,wherein the electrical properties and physical properties measured bysaid method are selected from the group consisting of specific weight,viscosity, temperature, dielectric constant and conductivity.
 9. Themethod of claim 8 , wherein said method simultaneously measures at leasttwo properties selected from the group consisting of specific weight,viscosity, temperature, dielectric constant and conductivity of eachliquid composition in the array of sample wells.
 10. The method of claim1 , further comprising the step of coating the tuning fork resonatorwith a material that modifies the characteristics of the tuning forkresonator.
 11. The method of claim 10 , wherein the material used insaid coating step is a functionality designed to change the resonatorresponse of the tuning fork resonator if a selected substance is presentin the fluid composition.
 12. The method of claim 11 , wherein saidfunctionality used in the coating step contains receptor molecules forattracting molecules of the selected substance in the fluid compositionto change the resonator response.
 13. The method of claim 1 , whereinthe placing step includes placing a plurality of tuning fork resonatorsin the fluid composition.
 14. The method of claim 13 , wherein thevarying step varies the frequency of the variable frequency input signalover a plurality of predetermined frequency ranges to obtain a pluralityof frequency dependent resonator responses from the plurality of tuningfork resonators.
 15. The method of claim 2 , further comprising the stepof coating at least one of said plurality of tuning fork resonators witha material that modifies the characteristics of said at least one tuningfork resonators.
 16. The method of claim 15 , wherein the material usedin said coating step is a functionality designed to change the resonatorresponse of said at least one tuning fork resonator of a selectedsubstance is present in the liquid composition associated with said atleast one tuning fork resonator.
 17. The method of claim 16 , whereinsaid functionality used in the coating step contains receptor moleculesfor attracting molecules of the selected substance in the liquidcomposition associated with said at least one tuning fork resonator. 18.A method for monitoring a change in a property of a liquid composition,the method comprising: placing the mechanical resonator in the liquidcomposition such that at least a portion of the mechanical resonator issubmerged in the liquid composition; applying a variable frequency inputsignal to a measurement circuit coupled with the mechanical resonator tooscillate the mechanical resonator; varying the frequency of thevariable frequency input signal over a predetermined frequency range toobtain a frequency-dependent resonator response of the mechanicalresonator; determining the property of the liquid based on themechanical resonator response; repeating the applying, varying, anddetermining steps over time; and monitoring over time a change in themechanical resonator response reflecting the change in property of theliquid composition.
 19. The method of claim 18 , the change monitored inthe monitoring step is a physical change in the liquid composition. 20.The method of claim 19 , wherein the physical change in the liquidcomposition is a liquid-to-solid state transformation of the liquidcomposition.
 21. The method of claim 18 , wherein the change monitoredin the monitoring step is a chemical transformation of the liquidcomposition.
 22. The method of claim 21 , wherein the chemicaltransformation monitored in the monitoring step is a polymerizationreaction.
 23. The method of claim 18 , wherein said method measures aproperty selected from the group consisting of specific weight,viscosity, temperature, dielectric constant and conductivity of theliquid composition and monitors said property over time.
 24. The methodof claim 23 , wherein said method simultaneously measures at least twoproperties selected from the group consisting of specific weight,viscosity, temperature, dielectric constant and conductivity of theliquid composition and monitors said properties over time.
 25. Themethod of claim 18 , further comprising the step of coating themechanical resonator with a material that modifies the characteristicsof the mechanical resonators.
 26. The method of claim 25 , wherein thematerial used in said coating step is a functionality designed to changethe resonator response of the mechanical resonator if a selectedsubstance is present in the liquid composition.
 27. The method of claim26 , wherein said functionality used in the coating step containsreceptor molecules for attracting molecules of the selected substance inthe liquid composition to change the resonator response.
 28. The methodof claim 18 , wherein the placing step includes placing a plurality ofmechanical resonators in the liquid composition.
 29. The method of claim28 , wherein each of said plurality of mechanical resonators has adifferent resonator response characteristic, and wherein the varyingstep varies the frequency of the variable input signal over a pluralityof frequency dependent resonator responses from the plurality ofmechanical resonators.
 30. The method of claim 18 , wherein themechanical resonator placed in the placing step is a multiple-moderesonator that can be operated in more than one mechanical mode, andwherein the varying step comprises varying the frequency of the variablefrequency input signal to obtain a plurality of frequency-dependentresonator responses corresponding to said more than one mechanical mode.31. An apparatus for measuring a property of a fluid composition,comprising: a tuning fork resonator; means for containing the fluidcomposition; a measurement circuit coupled with said tuning forkresonator, said measurement circuit having a signal generator forgenerating a variable frequency input signal to cause said tuning forkto oscillate; and a receiver coupled to the measurement circuit tooutput a frequency response of said tuning fork resonator.
 32. Theapparatus of claim 31 , wherein the fluid composition is a liquidcomposition, the apparatus further comprising: an array of tuning forkresonators; and an array of sample wells for holding a plurality ofliquid compositions, and wherein said measurement circuit and saidreceiver are coupled with said array of tuning fork resonators to obtaina frequency response associated with each of said plurality of liquidcompositions.
 33. The apparatus of claim 31 , wherein the tuning forkcomprises at least two tines, each tine including a quartz crystalcenter portion having at least two faces and an electrode on at leastone of the two faces of said quartz crystal center portion.
 34. Theapparatus of claim 31 , wherein said quartz crystal center portion ofeach tine has four faces and wherein four electrodes are connected tosaid quartz crystal center portion such that one electrode is coupled toeach face.
 35. The apparatus of claim 31 , further comprising a coatingmaterial on said tuning fork resonator that modifies the characteristicsof the tuning fork resonator.
 36. The apparatus of claim 35 , whereinsaid coating material is a functionality designed to change theresonator response of the tuning fork resonator if a selected substanceis present in the fluid composition.
 37. The apparatus of claim 36 ,wherein said functionality contains receptor molecules for attractingmolecules of the selected substance in the fluid composition to changethe resonator response.
 38. The apparatus of claim 31 , furthercomprising a plurality of tuning fork resonators, each tuning forkresonator having a different resonator response characteristic.
 39. Theapparatus of claim 38 , wherein each tuning fork has a differentfunctionality, each functionality designed to change the resonatorresponse of its associated tuning fork resonator if a selected substancecorresponding with the specific coating is present in the fluidcomposition.
 40. A method for measuring a property of a plurality ofliquid compositions using a plurality of mechanical resonators, themethod comprising: providing an array of sample wells; placing each ofsaid plurality of liquid compositions in a separate sample well; placingat least one of said plurality of mechanical resonators into at leastone of said sample wells such that at least a portion of the mechanicalresonator is submerged in its associated liquid composition; applying avariable frequency input signal to a measurement circuit coupled with atleast one of said plurality of mechanical resonators to oscillate saidat least one mechanical resonator in its associated liquid composition;varying the frequency of the variable frequency input signal over apredetermined frequency range to obtain a frequency-dependent resonatorresponse of the at least one mechanical resonator; and determining theproperty of the liquid based on the mechanical resonator response tomeasure a property of each liquid composition.
 41. The method of claim40 , further comprising the step of distinguishing between at least twoof said plurality of liquid compositions based on the mechanicalresonator response.
 42. The method of claim 40 , wherein the mechanicalresonator placed in the placing step is a thickness shear moderesonator.
 43. The method of claim 40 , wherein the mechanical resonatorplaced in the placing step is a tuning fork resonator.
 44. The method ofclaim 40 , further comprising placing a plurality of mechanicalresonators in the placing step.
 45. The method of claim 44 , wherein theplurality of mechanical resonators placed in the placing step areselected from the group consisting of tridents, cantilevers, torsionbars, length extension resonators, bimorphs, unimorphs, membraneresonators, surface acoustic wave devices, thickness share moderesonators, and tuning fork resonators.
 46. The method of claim 45 ,wherein each of the plurality of mechanical resonators placed in theplacing step is a different type of mechanical resonator from the othermechanical resonators in the plurality of mechanical resonators.
 47. Themethod of claim 40 , wherein the mechanical resonator placed in theplacing step is a multiple-mode resonator that can be operated in morethan one mechanical mode, and wherein the varying step comprises varyingthe frequency of the variable frequency input signal to obtain aplurality of frequency-dependent resonator responses corresponding tosaid more than one mechanical mode.
 48. The method of claim 40 , whereinthe mechanical resonator placed in the placing step is one selected fromthe group consisting of tridents, cantilevers, torsion bars, bimorphs,unimorphs, membrane resonators, and surface acoustic wave devices. 49.The method of claim 40 , wherein said method measures a physicalproperty of each liquid composition in the array of sample wells. 50.The method of claim 49 , wherein the physical property measured by saidmethod is selected from the group consisting of specific weight,temperature and viscosity.
 51. The method of claim 40 , wherein saidmethod measures an electrical property of each liquid composition in thearray of sample wells.
 52. The method of claim 51 , wherein theelectrical property measured by said method is selected from the groupconsisting of dielectric constant and conductivity.
 53. The method ofclaim 40 , wherein said method measures an electrical property and aphysical property of each liquid composition in the array of samplewells simultaneously.
 54. The method of claim 53 , wherein theelectrical properties and physical properties measured by said methodare selected from the group consisting of specific weight, viscosity,dielectric constant and conductivity.
 55. The method of claim 54 ,wherein said method simultaneously measures at least two propertiesselected from the group consisting of specific weight, viscosity,temperature, dielectric constant and conductivity of each liquidcomposition in the array of sample wells.
 56. The method of claim 40 ,further comprising the steps of: calibrating each of said tuning forkresonators against a standard liquid having known properties to obtaincalibration data; and determining the property of each liquidcomposition based on the calibration data.
 57. The method of claim 40 ,further comprising the step of coating at least one of said mechanicalresonators with a material that modifies the characteristics of themechanical resonator.
 58. The method of claim 57 , wherein the materialused in the coating step is a functionality designed to change theresonator response of the mechanical resonator if a selected substanceis present in the liquid composition.
 59. The method of claim 58 ,wherein the functionality used in the coating step contains receptormolecules for attracting molecules of the selected substance in theliquid composition to change the resonator response.
 60. A method formeasuring a property of a liquid composition flowing through a conduit,the method comprising: placing a mechanical resonator in the conduitsuch that at least a portion of the mechanical resonator will besurrounded by the fluid composition as the fluid composition flowsthrough the conduit; applying a variable frequency input signal to ameasurement circuit coupled with the mechanical resonator to oscillatethe tuning fork resonator; varying the frequency of the variablefrequency input signal over a predetermined frequency range to obtain afrequency-dependent resonator response of the mechanical resonator; anddetermining the property of the fluid composition based on themechanical resonator's response.
 61. The method of claim 60 , furthercomprising the steps of: calibrating the mechanical resonator against astandard fluid having known properties to obtain calibration data; anddetermining the property of the fluid composition based on thecalibration data.
 62. The method of claim 60 , wherein the mechanicalresonator placed in the placing step is a thickness shear moderesonator.
 63. The method of claim 60 , wherein the mechanical resonatorplaced in the placing step is a tuning fork resonator.
 64. The method ofclaim 60 , further comprising placing a plurality of mechanicalresonators in the placing step.
 65. The method of claim 64 , wherein theplurality of mechanical resonator s placed in the placing step areselected from the group consisting of tridents, cantilevers, torsionbars, length extension resonators, bimorphs, unimorphs, membraneresonators, surface acoustic wave devices, thickness share moderesonators, and tuning fork resonators.
 66. The method of claim 65 ,wherein each of the plurality of mechanical resonators placed in theplacing step is a different type of mechanical resonator from the othermechanical resonator in the plurality of mechanical resonators.
 67. Themethod of claim 60 , wherein the mechanical resonator placed in theplacing step is a multiple-mode resonator that can be operated in morethan one mechanical mode, and wherein the varying step comprises varyingthe frequency of the variable frequency input signal to obtain aplurality of frequency-dependent resonator responses corresponding tosaid more than one mechanical mode.
 68. The method of claim 60 , whereinthe mechanical resonator placed in the placing step is one selected fromthe group consisting of tridents, cantilevers, torsion bars, lengthextension resonators, bimorphs, unimorphs, membrane resonators, andsurface acoustic wave devices.
 69. The method of claim 60 , wherein saidmethod measures a physical property of the fluid composition flowingthrough the conduit.
 70. The method of claim 69 , wherein the physicalproperty measured by said method is selected from the group consistingof specific weight, temperature and viscosity.
 71. The method of claim60 , wherein said method measures an electrical property of the fluidcomposition flowing through the conduit.
 72. The method of claim 71 ,wherein the electrical property measured by said method is selected fromthe group consisting of dielectric constant and conductivity.
 73. Themethod of claim 60 , wherein said method measures simultaneously anelectrical property and a physical property of the fluid compositionflowing through the conduit.
 74. The method of claim 73 , wherein theelectrical properties and physical properties measured by said methodare selected from the group consisting of specific weight, viscosity,temperature, dielectric constant and conductivity.
 75. The method ofclaim 74 , wherein said method simultaneously measures at least twoproperties selected from the group consisting of specific weight,viscosity, temperature, dielectric constant and conductivity of thefluid composition flowing through the conduct.
 76. The method of claim60 , further comprising the step of coating the mechanical resonatorwith a material to modify the characteristics of the mechanicalresonators.
 77. The method of claim 76 , wherein the material used insaid coating step is a functionality designed to change the resonatorresponse of the mechanical resonator if a selected substance is presentin the fluid composition to change the resonator response.
 78. Themethod of claim 77 , wherein said functionality used in the coating stepcontains receptor molecules for attracting molecules of the selectedsubstance in the liquid composition.